The present invention relates to an apparatus for measuring electric conductivity, and specifically, relates to an apparatus for measuring electric conductivity capable of measuring electric conductivity of a substance to be measured stably and accurately at all times.
Electric conductivity is especially employed as a scale to measure a concentration of ions capable of migrating in an aqueous solution, and an apparatus for measuring electric conductivity is used to measure ion concentrations in many kinds of aqueous solutions. An apparatus for measuring electric conductivity, generally, determines an increase or a decrease of the ion concentration of an aqueous solution by measuring the resistance of the aqueous solution existing between an electric conductivity detection electrode and an electric current supply electrode connected to a power source.
Namely, a conventional apparatus for measuring electric conductivity is constituted, for example, as depicted in FIG. 9. In an apparatus for measuring electric conductivity 101 shown in FIG. 9, a power source electrode 104 and a detection electrode 105 for detecting electric conductivity are disposed apart from each other relative to a fluid 103 to be measured which flows into a measurement tube 102 or which is stored in the measurement tube 102. An AC constant voltage is applied to the power source supply electrode 104, for example, from a power source (not shown) through an amplifier 106. An electric current from the detection electrode 105 for detecting electric conductivity is output through an electric current amplifier 107, and is served for measuring electric conductivity. The measurement tube 102 is composed of an insulation material (for example, a polyvinyl chloride tube) at at least above-described portion for measuring electric conductivity, but the measurement tube is usually in a substantially grounded condition (a grounded point 108) at any position of extending portion of the tube.
In the apparatus for measuring electric conductivity 101 thus constructed, since a resistance corresponding to an electric conductivity of the fluid 103 to be measured exists between the electrodes 104 and 105, a small electric current flowing from the power source electrode 104 to the electric conductivity detection electrode 105 via the resistance is amplified by the electric current amplifier 107, and an output signal therefrom is determined as a value corresponding to the electric conductivity of the fluid to be measured.
However, in the apparatus for measuring electric conductivity 101 having such a constitution, since the measurement tube 102 is substantially grounded at any position of the extending portion thereof, an electric current from the electric conductivity detection electrode 105 flows into the electric current amplifier 107, and at the same time, a smaller electric current flows into the grounded point 108. Namely, from the electric conductivity detection electrode 105, a current leakage occurs more or less, independently of the electric current for measuring electric conductivity.
Since the position of the grounded point 108 is not fixed, and therefore the resistance between the electrode 105 and the grounded point 108 varies, and since a small electric current flowing between them flows into the electric current amplifier 107 via the earth, the leaked electric current flowing from the-above-described electrode 105 to the grounded point 108 is a factor varying the electric current detected from the electric conductivity detection electrode 105. Therefore, the existence of such a leaked electric current causes the measurement accuracy of the electric conductivity to be reduced. Further, the variation of the leaked electric current induces not only reduction of the measurement accuracy of the electric conductivity but also scattering of measured data of the electric conductivity.
Accordingly, it is an object of the present invention to provide an apparatus for measuring electric conductivity capable of measuring electric conductivity with a high accuracy by measuring electric conductivity of a substance to be measured stably at a condition with no variability at all times.
Further, it is another object to measure electric conductivity stably and accurately especially through the entire measuring period even when organic substances are contained in a substance to be measured, while making the above-described high-accuracy measurement possible.
To accomplish the above-described objects, the present invention relates to an apparatus for measuring electric conductivity comprising three electrodes brought into contact with a substance to be measured, the three electrodes including a detection electrode for detecting electric conductivity of the substance to be measured, and two AC current supply electrodes disposed on both sides of the electric conductivity detection electrode with respective distances, an AC current of the same phase being applied to the two AC current supply electrodes (a first apparatus for measuring electric conductivity). A substance to be measured is generally an aqueous solution, but a gaseous or slurry-like substance can also be measured.
In this first apparatus for measuring electric conductivity, although it is preferred that a constant voltage with the same electric potential is applied to the above-described two AC current supply electrodes, the potentials of the voltages applied to the two AC current supply electrodes may be different from each other. However, even in the latter case, the voltages applied to the respective AC current supply electrodes are to be predetermined constant voltages.
Further, an apparatus for measuring electric conductivity according to the present invention comprises three electrodes brought into contact with a substance to be measured, the three electrodes including a detection electrode for detecting electric conductivity of the substance to be measured, an AC current supply electrode disposed on one side of the electric conductivity detection electrode with a distance, and a grounded electrode disposed on the other side of the electric conductivity detection electrode with a distance (a second apparatus for measuring electric conductivity).
In this second apparatus for measuring electric conductivity, it is preferred that a constant voltage is applied to the above-described AC current supply electrode.
In the first and second apparatuses for measuring electric conductivity as described above, when organic substances and the like are contained in a substance to be measured, advantage can be taken of the photocatalytic activity of titanium oxide which decomposes organic substances or the super-hydrophilicity of titanium oxide in order to efface the influence on the measurement of electric conductivity exerted by adhesion or adsorption of organic substances to the electrode surfaces.
Namely, each of the above-described three electrodes can be constructed so that its electrode surface is formed by a titanium oxide layer provided on a surface of an electrode body made of a conductive metal. It is preferred that light irradiating means is disposed against the titanium oxide layer to exhibit a photocatalytic activity on the titanium oxide layer. For example, a construction can be employed, wherein the apparatus has a space for storing a substance to be measured defined between respective electrode surfaces of the above-described three electrodes, and light irradiating means that irradiates light onto the respective electrode surfaces.
In these apparatuses for measuring electric conductivity, it is preferred that the light irradiated by the above-described light irradiating means has a wavelength which brings about a photocatalytic activity of the titanium oxide layer. For example, light with a wavelength from about 300 to about 400 nm can be employed. As means for irradiating light, a light source composed of means for irradiating ultraviolet rays and the like such as a black light may be directly employed, and a light guiding material (for example, an optical fiber, or tube and the like comprising a light guiding raw material) to guide light from a light source provided as means for irradiating light may also be employed. Further, the light from a light guiding material may be added to light irradiated directly from a light source.
Further, the above-described space for storing a substance to be measured may be defined by a light transmitting material, and it may be constituted so that the light from the light irradiating means is irradiated onto an electrode surface through the light transmitting material (for example, glass). In this case, if a titanium oxide coating layer capable of transmitting light is provided on the surface of the light transmitting material at its side facing the space for storing a substance to be measured (a surface in contact with solution), adhesion of organic substances and the like to this surface of the light transmitting material can be prevented by super-hydrophilicity and organics decomposition property ascribed to the titanium oxide layer.
Further, the above-described electrode can be produced by, for example, the following method. Namely, a method can be employed wherein an electrode surface consists of a titanium oxide layer formed by a surface treatment such as sputtering, plating or the like on a surface of an electrode body made of a conductive metal. Alternatively, a method can also be employed wherein an electrode surface made of a titanium oxide layer is formed by providing oxygen to a surface of an electrode body made of titanium. As the method for forming a titanium oxide layer by providing oxygen, a method based on air oxidation other than a method utilizing electrolysis can be employed.
In the first apparatus for measuring electric conductivity according to the present invention as described above, the electric conductivity detection electrode is disposed between two AC current supply electrodes, and an AC current of the same phase is applied to the two AC current supply electrodes. By this constitution, the electric conductivity detection electrode is electrically shielded by the two AC current supply electrodes against any grounded portion existing outside of the positions where these three electrodes are disposed. Therefore, no resistance exists between the electric conductivity detection electrode and an outside grounded portion, and substantially any leaked electric current does not flow therebetween. As a result, an electric current for measuring electric conductivity can be obtained stably at all times from the electric conductivity detection electrode, and any scattering of the measured data of the electric conductivity can be prevented, and a high-accuracy measurement of the electric conductivity becomes possible.
Further, in the second apparatus for measuring electric conductivity according to the present invention, since an AC current is applied to only one electrode of the two electrodes disposed on both sides of the electric conductivity detection electrode and the other electrode is grounded, the portion between these two electrodes is set in a formation of so-called resistive division which is created by the electric conductivity detection electrode disposed therebetween. And, since a constant voltage is applied to the above-described one electrode, and the potential of the other grounded electrode is always zero, the resistance between the electric conductivity detection electrode and the grounded electrode can be fixed at a constant value substantially having no variation. Therefore, even if the measurement tube for the electric conductivity is in a grounded condition at any position of its extending portion, because the above-described other electrode is forcibly grounded before reaching the grounded point of the measurement tube, and the electric potential is focibly reduced to zero at the position of the electrode, there is no room to allow a resistance of high variability to enter between the electric conductivity detection electrode and the grounded point of the measurement tube. Consequently, an electric current for measuring electric conductivity can be obtained from the electric conductivity detection electrode stably at all times, and any scattering of the measured data of the electric conductivity can be prevented, and a high-accuracy measurement of the electric conductivity becomes possible.
Thus, in the apparatus for measuring electric conductivity according to the present invention, since the AC current supply electrodes are disposed, or the AC current supply electrode and the grounded electrode are disposed on the both sides of the detection electrode for detecting electric conductivity, and the electric conductivity detection electrode between both electrodes can be electrically shielded adequately against an outside grounded portion, any disturbance can be prevented from being generated in an electric current extracted from the electric conductivity detection electrode, and therefore, the electric conductivity of a substance to be measured can be measured with a high accuracy under the measurement condition being always stabilized.
Further, in such first and second apparatuses for measuring electric conductivity, if the electrodes having titanium oxide layers on their surfaces are used, the photocatalytic activity of titanium oxide is exhibited by irradiating light with an appropriate wavelength (for example, an ultraviolet ray) to the titanium oxide layers, organic substances in contact with the titanium oxide layers or existing in the water near the layers are decomposed, and the adhesion or adsorption thereof to the titanium oxide layers is prevented. Further, since at first water films are formed on the electrode surfaces by the super-hydrophilicity that the titanium oxide layers have, even if the decomposition of the organic substances is delayed, the adhesion itself can be suppressed. Therefore, the electrode surfaces are always maintained at a desirable surface condition without adhesion or adsorption of the organic substances, and the desirable surface condition is maintained stably at all times without requiring any periodical cleaning and the like. Therefore, without requiring a periodical cleaning of the electrode surfaces, electric conductivity can be measured stably and accurately at all times, and repeatability of the accuracy of the measurement can be ensured with no problems.